This study is concerned with those underlying physical chemical factors which are responsible for lipid interactions in biological membranes and for the lack of lipid interaction and resulting phase separation of lipids in aqueous systems. The major objective of this study is to study quantitatively the interactions occurring between fatty acid chains when they are situated at interfaces and in contact with water. A large number of fatty acids differing in: chain length, presence of double, triple bonds, and hydroxyl groups; and as to the number of such groups on the chain will be studied. Studies will be carried out with insoluble monomolecular films spread on an aqueous surface. By measuring surface tension changes with compression of the film molecules from very dilute states to tightly packed states it will be possible to measure the free energies, enthalpies and entropis of compression and hence, the energetics associated with chain-chain and chain-water interactions. Treating mixed films of dissimilar fatty acids as two-dimensional solutions will allow measurement of their interaction energies and any tendency for phase separation. Of particular interest will be the influence of polyunsaturation on the temperature dependence of such behavior. The simplicity of the technique and the very small amounts of material required make this a unique tool for answering fundamental facial structures and biological membranes.